This sentence does not contain a comma where necessary. The phrase "While flipping through books" is an example of an introductory phrase, explaining information necessary to the context of the sentence, but outside of the main sentence, grammatically. These types of phrases must be followed by a comma in order to separate them from the rest of the sentence. The correct answer choice is, "While flipping through books, Masha noticed."

The words "gentle" and "calming" are coordinate adjectives: adjectives that modify the same noun separately, but are not separated with a conjunction. Coordinate adjectives need to be separated with a comma. The correct answer choice is, "She enjoyed the gentle, calming breeze."

In a parallelism with three objects, it is generally correct to use a comma after the first two objects. All of the other answer choices contain comma splice errors or errors with the use of possessive apostrophes.

First, "can not" and "cannot" are both considered correct in modern English. Second, the word however may be used after a semicolon to join two independent clauses. In this case the word however is being used simply as an adverb modifying the word "compelling". The incorrect answer choices create comma splice errors or use the semicolon incorrectly. The correct answer choice simply omits the comma after however and places it at the end of the dangling modifier "However...argument".

The problem here is not one of modifying the verb; rather, it is a matter of correctly using a comma to set off the dangling modifier "A California native." The correct syntax is Dangling Modifier / Comma/ Modified Noun. Eliminate the answer choices that create comma splice errors and think of the answer choices as they relate to the sentence as a whole. 

When using the conjunctive adverb "therefore" at the beginning of a sentence, it is appropriate to use a comma to set it off. All of the other answer choices demonstrate incorrect comma usage and comma splice errors.

Placing a comma after "and" results in a comma splice, separating a compound object. A comma is appropriate after the adverb "today." No other comma is needed in the sentence. In fact, using other commas results in gross mechanical errors, changing the conjunction "and" to "but" also incorrectly modifies the compound object "AIDS and breast cancer".

This sentence does not contain a comma where necessary. The words "sleek" and "modern" are coordinate adjectives: adjectives that modify the same noun separately, but are not separated with a conjunction. Coordinate adjectives need to be separated with a comma. The correct answer choice is, "The sleek, modern building was a marvel."

This sentence contains a comma where one is not necessary. While the words "great" and "courageous" are coordinate adjectives, and, as such, should be separated by a comma after the word "great," there is no need whatsoever for a comma after the word "courageous." The second comma should be omitted. The correct choice is, "The statue of the great, courageous hero stood."

This sentence does not contain a comma where necessary. The phrase "which the company ordered is an example of a relative clause, which gives more information about its antecedent: equipment. Any such phrase that occurs in the middle of a sentence must be introduced and completed with a comma. The correct answer choice is, "equipment, which the company ordered, seemed to have."